The present invention relates to both a system and a process for activating the hydraulic service-brake system of a road vehicle equipped with an anti-lock system (ABS) working on the return-flow principle and with a drive-slip control device (ASR) working on the principle of the differential wheel brake-activation of a driven vehicle wheel tending to spin. The process according to the present invention obtains as rapid a situation-related response as possible of the brake of the driven vehicle wheel when the latter is tending to spin. To this end, when the drive-slip control commences, the main brake-line of the brake circuit of the driven vehicle wheels is shut off from the static master cylinder of the brake system; for the build-up of brake pressure in the brake circuit of the driven vehicle wheels, the return pump provided for this brake circuit and belonging to the anti-lock system configured to work on the return-flow principle is used.
In order to set a specific high pressure level as early as at the commencement of the control, that portion of the main brake-line an its portions branching off towards the wheel brakes, via which the ASR-function control valve is connected to the brake-pressure regulating valves which are assigned individually to the driven vehicles wheels and by way of which brake-pressure build-up, holding and reduction phases both of the drive-slip and of the anti-lock control can be controlled even before the ASR-control commences, is charged to a specific high pressure, the brake-pressure regulating valves are previously changed to their blocking positions, and the return pump is simultaneously activated as a pressure source. This activation of the return pump and changeover of the brake-pressure regulating valves takes place at a moment when the situation requiring control begins to emerge. That is, an electronic control unit detects, via output signals from wheel-speed sensors assigned individually to the vehicle wheels, that the wheel acceleration and/or the drive-slip is beginning to increase and threshold values of the acceleration and/or of the drive-slip are being exceeded. These signals, although not yet requiring a cut-in of the control, nevertheless indicate that control will very soon be needed.
An activation process is shown in from DE 3,839,178 A1. Although this known process is reasonably satisfactory in serving the purpose of ensuring a rapid control response, nevertheless it has the disadvantage that those parts of the main brake-line of the brake circuit of the driven vehicle wheels utilized, as it were, as additional pressure accumulators are pressurized to a very high pressure. Should the control still not commence immediately, but only relatively late, those parts are therefore subjected to very high load. Pressure relief valves sized for a high pressure level and which involve a comparatively high outlay and susceptible to faults consequently thus have to be provided. The brake system has to be protected against damage to the ASR-control valve, by means of which the master cylinder of the brake system is shut off from the main brake-line in the control mode. In other words, central valves provided in the brake master cylinder have to be designed for a high load-bearing capacity so that they cannot be damaged in the event of a pressure surge which could occur as a result of damage to the ASR-control valve, thus necessitating a construction of the brake unit which involves a high outlay.
In addition, the storage capacity of the brake-line portions connected between the ASR-function control valve and the brake-pressure regulating valves is limited and therefore is often insufficient to guarantee, with the control response, also an immediate effective application of the wheel brake or wheel brakes subjected to the control. This can occur especially when the lifting play of the wheel-brake cylinders has become relatively large, which occurs regularly when the brake system has been actuated with only moderate force over a plurality of braking operations, and consequently the pistons of the wheel-brake cylinders have been displaced only within the elastic deformability of their sealing rings acting at the same time as return elements. As a result, relatively long displacement travels of the wheel-cylinder pistons are first necessary in order to apply the brakes effectively. In such instances, an undesirable sluggish response behavior of the drive-slip control then has to be accepted.
An object of the present invention is, therefore, to provide an activation process of the aforementioned type which guarantees both a reliable, rapid response of the drive-slip control independently of the particular lifting play of the brake system and a brake system which allows an automatically controlled use of the process on a road vehicle.
Another object of the present invention is to provide a brake system for a road vehicle suitable for carrying out the improved process.
These objects have been achieved in accordance with the process and system of the present invention, by a process in which, in an operating situation of the vehicle which can be detected by output signals from wheel-speed sensors assigned to driven and nondriven vehicle wheels and which requires a drive-slip control, even before the control responds as a result of the activation of one or more brakes of driven vehicle wheels, at least the brake-line branches leading to the wheel brakes and the wheel brakes of the driven vehicle wheels, connected thereto via brake-pressure regulating valves provided both for the anti-lock control mode and for the drive-slip control mode, are loaded by the outlet pressure of a pump working at a low pressure level of between 10 bar and 20 bar, and at the latest at the activation of this pump, the brake circuit of the driven vehicle wheels is shut off by the ASR-function control valve from the outlet-pressure space assigned to this brake circuit, of the system brake unit of the brake system. In an operating phase of the vehicle initiating a braking operation, at least the brake circuit of the driven vehicle wheels is shut off by the ASR-function control valve and/or the brake circuit of the non-driven vehicle wheels is/are shut off from the respective associated outlet-pressure space of the brake unit, and the brake-line branches leading to the wheel brakes of this brake circuit and the wheel brakes are loaded with the outlet pressure of the low-pressure pump.
The low-pressure loading of the brake circuit of the driven vehicle wheels, even before the drive-slip control, which always commences with a pressure loading of at least one of the wheel brakes of the driven vehicle wheels, ensures that the wheel brake of a vehicle wheel to be subjected to the control at a slightly later moment is already being applied weakly when the control commences. Thus, a pressure build-up by means of the high-pressure pump, initiated thereafter for the control, can become effective immediately, with the result that a very rapid and sensitive response of the control becomes possible.
At the same time, the metering of pressure to the wheel brakes of the driven vehicle wheels can be appropriately controlled by the assigned brake-pressure regulating valves such that first only the wheel brake of that vehicle wheel for which a response of the control is to be expected first is loaded by the outlet pressure of the low-pressure pump. This can be achieved in that the wheel brake of the other driven vehicle wheel is shut off by the brake-pressure regulating valve or valves assigned thereto from the low-pressure pump in the introductory phase of the preliminary pressure build-up and is likewise connected to the low pressure source only after a short delay time of 100 to 200 ms. After that delay time, its wheel brake too is initially applied with a moderate force which still exerts no, or at least no appreciable, braking effect on this wheel. The control signals necessary for this control, in which the brake-pressure regulating valves are solenoid valves, are generated in control-related sequence and combination by the electronic ASR- and ABS-control unit.
A currently preferred embodiment of the process according to the present invention ensures a rapid response of the brakes of the vehicle even during a normal braking, i.e. one which is not subjected to control and which is initiated and controlled by the pedal actuation of the brake unit of the brake system. Especially when all the wheel brakes of the vehicle are loaded with the assistance of the low-pressure pump, this embodiment achieves a minimization of the idle travel of the brake pedal and guarantees that the greatest possible part of the available pedal actuation stroke can be utilized for exerting the brake pressure. This is of considerable advantage in terms of the maximum obtainable braking forces.
Insofar as the process according to the present invention is used for executing control cycles of the drive-slip control, it is, of course, also possible to achieve an additional response sensitivity of the control. Specifically, after a period of time after the expiration of which, the wheels brakes can be assured to be applied weakly as a result of the low-pressure loading, the associated brake pressure regulating valves are changed over to the blocking position and, in a manner similar to the conventional activation process discussed above, the brake-line portions and branches connecting the ASR-function control valve to the brake-pressure regulating valves of the brake circuit of the driven vehicle wheels are already being loaded by the high outlet pressure of the return pump, utilized as a brake-pressure source, of this brake circuit. This is then applied to the brake-pressure regulating valves even when these are being switched back into the pressure build-up position for the purpose of activating the wheel brake or wheel brakes.
According to another feature of the present invention, a non-return valve is provided to ensure that braking into a drive-slip control operation can be carried out free of delay even when the ASR-function control valve, by means of which the brake unit is shut off from the brake circuit of the driven wheels during the drive-slip control phases, is not immediately changed over at the start of braking into its functional position connecting the brake unit to the main brake-line. A slight delay in the changeover of the ASR-function control valves into its relation to the ASR-function position is even beneficial, in order, if a very high brake pressure was fed into the wheel brake or wheel brakes of the brake circuit of the driven vehicle wheels in the AS-control phase and the driver would like to select a relatively low pressure by way of the brake unit, to prevent reactions on the brake unit which the driver would detect as a recoil force on the brake pedal and which could even lead to an unnecessarily high stress on gaskets within the brake unit.
According to yet another feature of the present invention, even during normal braking, an advantageously rapid response of all the wheel brakes of the vehicle and a minimization of the lifting play or idle travel of the brake pedal are ensured, only after which does the exertion of brake pressure proportional to the actuating force commence.
Other features of the present invention, which can be used alternately or in combination, include an isolating valve which is suitable for shutting off the brake circuit of the non-driven vehicle wheels from the brake unit at the start of braking and the combined form of which affords additional safety in respect of a function-related changeover of the isolating valve.
If only hydraulic control is provided for such an isolating valve, a valve configuration according to another aspect of the present invention ensures in a simple way that, in the introductory phase of a braking operation, first the brake circuit of the driven vehicle wheels and only thereafter the brake circuit of the non-driven vehicle wheels is activated. This feature has the advantages that, in a vehicle driven via the rear wheels, the wheel brakes of the rear-axle brake circuit are first utilized to a greater extent for exerting the brake force than the front-wheel brakes in the introductory phase of braking, as a result, a relatively careful treatment of the front-wheel brakes can be achieved without a loss of exertion of brake force having to be accepted.
It is another advantage of the present invention also with regard to the brake circuit of the non-driven vehicle wheels that a bypass flow path parallel to the isolating valve and with a non-return valve is provided, via which brake pressure can be fed into the brake circuit in the event of a malfunction of the isolating valve.
As a result of a mode of the activation of the low-pressure pump and changeover of the ASR-function control valve and, if appropriate, of the isolating valve into their blocking positions according to still a further aspect of the present invention, a preliminary pressure loading of the brake circuits can be initiated even before the driver actuates the brake pedal. As an indication that braking is to take place, the logical linkage of a signal characteristic of the basic position of the accelerator pedal with a signal characteristic of a minimum driving speed is used because this signal combination occurs with predominant probability only when the driver takes his foot off the accelerator pedal in order subsequently to brake. It is possible in this way t gain up to 200 ms which can be utilized for the preliminary pressure loading of the brake circuits.
In the same situation, an even earlier activation of the low-pressure pump and of the ASR-function control and isolating valves is possible if, instead of a signal characteristic of the end position, a signal indicating a change of position of the accelerator pedal is generated, as provided according to another feature of the system of the present invention.
At all events, it is expedient if an activation of the low-pressure pump and of the ASR-function control valve and, if appropriate, of the isolating valve, which is based in this way on indications, is canceled again after a time period tv. A typical duration of this time period is between 200 ms and 500 ms.